Dual-Criteria Decision Analysis by Multiphotonic Effects in Nanostructured ZnO

Simultaneous interrogation of pump and probe beams interacting in ZnO nanostructures of a two-wave mixing is proposed for dual-path data processing of optical signals by nonlinear optical effects. An enhancement in third-order nonlinear optical properties was exhibited by Al-doped ZnO thin films. Multiphoton absorption and nonlinear refraction were explored by the z-scan technique at 532 nm with nanosecond pulses. The evolution of the optical Kerr effect in the ZnO thin films was analyzed as a function of the incorporation of Al in the sample by a vectorial two-wave mixing method. Electrical and photoconductive effects were evaluated to further characterize the influence of Al in the ZnO solid samples. Potential applications of nonlinear optical parameters for encoding and encrypting information in light can be envisioned.

Photoelectric Properties of GaS1-xSex (0 ≤ x ≤ 1) Layered Crystals

In this study, the photoelectric properties of a complete series of GaS1-xSex (0 ≤ x ≤ 1) layered crystals are investigated. The photoconductivity spectra indicate a decreasing bandgap of GaS1-xSex as the Se composition x increases. Time-resolved photocurrent measurements reveal a significant improvement in the response of GaS1-xSex to light with increasing x. Frequency-dependent photocurrent measurements demonstrate that both pure GaS crystals and GaS1-xSex ternary alloy crystals exhibit a rapid decrease in photocurrents with increasing illumination frequency. Crystals with lower x exhibit a faster decrease in photocurrent. However, pure GaSe crystal maintains its photocurrent significantly even at high frequencies. Measurements for laser-power-dependent photoresponsivity and bias-voltage-dependent photoresponsivity also indicate an increase in the photoresponsivity of GaS1-xSex as x increases. Overall, the photoresponsive performance of GaS1-xSex is enhanced with increasing x, and pure GaSe exhibits the best performance. This result contradicts the findings of previous reports. Additionally, the inverse trends between bandgap and photoresponsivity with increasing x suggest that GaS1-xSex-based photodetectors could potentially offer a high response and wavelength-selectivity for UV and visible light detection. Thus, this work provides novel insights into the photoelectric characteristics of GaS1-xSex layered crystals and highlights their potential for optoelectronic applications.

Ferroelectric Properties of Polymer-Semiconductor Hybrid Material or Composite under Optical Excitation

Polymer-semiconductor hybrid materials or composites have been investigated with respect to their microstructure, optical, photoconductive, and ferroelectric properties. For this purpose, either CdSe quantum dots or (Cd:Zn)S microparticles were dispersed in poly(vinylidenefluoride-trifluoroethylene) solution and hot pressed to films. In both material systems, the electrical conductivity and the polarization behavior could be controlled by the intensity of the optical excitation. The simultaneous high optical transparency of the CdSe quantum-dot-based hybrid materials makes them particularly interesting for applications in the field of flexible, high-resolution sensors.

Observation of Ultrahigh Photoconductivity in DNA-MoS2 Nano-Biocomposite

A nano-biocomposite film with ultrahigh photoconductivity remains elusive and critical for bio-optoelectronic applications. A uniform, well-connected, high-concentration nanomaterial network in the biological matrix remains challenging to achieve high photoconductivity. Wafer-scale continuous nano-biocomposite film without surface deformations and cracks plays another major obstacle. Here ultrahigh photoconductivity is observed in deoxyribonucleic acid-molybdenum disulfide (DNA-MoS2) nano-biocomposite film by incorporating a high-concentration, well-percolated, and uniform MoS2 network in the ss-DNA matrix. This is achieved by utilizing DNA-MoS2 hydrogel formation, which results in crack-free, wafer-scale DNA-MoS2 nano-biocomposite films. Ultra-high photocurrent (5.5 mA at 1 V) with a record-high on/off ratio (1.3 × 106) is observed, five orders of magnitude higher than conventional biomaterials (≈101) reported so far. The incorporation of the Wely semimetal (Bismuth) as an electrical contact exhibits ultrahigh photoresponsivity (2.6 × 105 A W-1). Such high photoconductivity in DNA-MoS2 nano-biocomposite could bridge the gap between biology, electronics, and optics for innovative biomedicine, bioengineering, and neuroscience applications.

Pressure-Driven Polymorphic Transition, Emergent Insulator-To-Metal Transition, and Photoconductivity Switching in Violet Phosphorus

Polymorphic phase transition is an essential phenomenon in condensed matter that the physical properties of materials may undergo significant changes due to the structural transformation. Phase transition has thus become an important means and dimension for regulating material properties. Herein, this study demonstrates the pressure-induced multi-transition of both structure and physical properties in violet phosphorus, a novel phosphorus allotrope. Under compression, violet phosphorus undergoes sequential polymorphic phase transitions. Concomitant with the first phase transition, violet phosphorus exhibits emergent insulator-metal transition, superconductivity, and dramatic switching from positive to negative photoconductivity. Remarkably, the resistance of violet phosphorus shows a sudden drop of around 107 along with the phase transition. In addition, piezochromism from translucent red to opaque black and suppression of photoluminescence are observed upon compression. Of particular interest is that the sample irreversibly transforms into black phosphorus with a pronounced discrepancy in physical properties from the pristine violet phosphorus after decompression. The abundant polymorphic transitions and property changes in violet phosphorus have significant implications for designing novel pressure-responsive electronic/optoelectronic devices and exploring concealed polymorphic transition materials.

Charge Carrier Mobility in Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine Composites with Electron Acceptor Molecules

Polymer composites based on poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (poly-TPD) with PCBM and copper(II) pyropheophorbide derivative (Cu-PP) were developed. In thin films of the poly-TPD and Cu-PP composites, the charge carrier mobility was investigated for the first time. In the ternary poly-TPD:PCBM:Cu-PP composite, the electron and hole mobilities are the most balanced compared to binary composites and the photoconductivity is enhanced due to the sensitization by Cu-PP in blue and red spectral ranges. The new composites are promising for use in the development of photodetectors.

Characterization of LaFeO3 Dielectric Ceramics Produced by Spark Plasma Sintering

Commercially available LaFeO3 powder was processed using the spark plasma sintering (SPS) technique. The results of the dielectric measurement showed high permittivity, but this was strongly frequency-dependent and was also accompanied by a high loss tangent. The chemical purity of the powder and changes induced by the SPS process influenced the stability of the dielectric parameters of the bulk compacts. A microstructure with a homogeneous grain size and a certain porosity was produced. The microhardness of the sintered LaFeO3 was rather high, about 8.3 GPa. All the results are in reasonable agreement with the literature related to the production of LaFeO3 using different techniques. At frequencies as low as 100 Hz, the material behaved like a colossal permittivity ceramic, but this character was lost with the increasing frequency. On the other hand, it exhibited persistent DC photoconductivity after illumination with a standard bulb.

Photonic Physical Reservoir Computing with Tunable Relaxation Time Constant

Recent years have witnessed a rising demand for edge computing, and there is a need for methods to decrease the computational cost while maintaining a high learning performance when processing information at arbitrary edges. Reservoir computing using physical dynamics has attracted significant attention. However, currently, the timescale of the input signals that can be processed by physical reservoirs is limited by the transient characteristics inherent to the selected physical system. This study used an Sn-doped In2 O3 /Nb-doped SrTiO3 junction to fabricate a memristor that could respond to both electrical and optical stimuli. The results show that the timescale of the transient current response of the device could be controlled over several orders of magnitude simply by applying a small voltage. The computational performance of the device as a physical reservoir is evaluated in an image classification task, demonstrating that the learning accuracy could be optimized by tuning the device to exhibit appropriate transient characteristics according to the timescale of the input signals. These results are expected to provide deeper insights into the photoconductive properties of strontium titanate, as well as support the physical implementation of computing systems.

Temperature-Dependent Conduction and Photoresponse in Few-Layer ReS2

The electrical behavior and the photoresponse of rhenium disulfide field-effect transistors (FETs) have been widely studied; however, only a few works have investigated the photocurrent as a function of temperature. In this paper, we perform the electrical characterization of few-layer ReS2-based FETs with Cr-Au contacts over a wide temperature range. We exploit the temperature-dependent transfer and output characteristics to estimate the effective Schottky barrier at the Cr-Au/ReS2 interface and to investigate the temperature behavior of parameters, such as the threshold voltage, carrier concentration, mobility, and subthreshold swing. Through time-resolved photocurrent measurements, we show that the photocurrent increases with temperature and exhibits a linear dependence on the incident light power at both low and room temperatures and a longer rise/decay time at higher temperatures. We surmise that the photocurrent is affected by the photobolometric effect and light-induced desorption of adsorbates which are facilitated by the high temperature and the low pressure.

Bottom-Up Photosynthesis of an Air-Stable Radical Semiconductor Showing Photoconductivity to Full Solar Spectrum and X-Ray

Single-component semiconductors with photoresponse to full solar spectrum are highly desirable to simplify the device structure of commercial photodetectors and to improve solar conversion or photocatalytic efficiency but remain scarce. This work reports bottom-up photosynthesis of an air-stable radical semiconductor using BiI3 and a photochromism-active benzidine derivative as a photosensitive functional motif. This semiconductor shows photoconductivity to full solar spectrum contributed by radical and non-radical forms of the benzidine derivative. It has also the potential to detect X-rays because of strong X-ray absorption coefficient. This finding opens up a new synthetic method for radical semiconductors and may find applications on extending photoresponsive ranges of perovskites, transition metal sulfides, and other materials.

Extreme γ-Ray Radiation Tolerance of Spectrometer-Grade CsPbBr3 Perovskite Detectors

The perovskite compound CsPbBr3 has recently been discovered as a promising room-temperature semiconductor radiation detector, offering an inexpensive and easy-to-manufacture alternative to the current benchmark material Cd1-x Znx Te (CZT). The performance of CsPbBr3 sensors is evaluated under harsh conditions, such as high radiation doses often found in industrial settings and extreme radiation in space. Results show minimal degradation in detector performance after exposure to 1 Mrad of Co-60 gamma radiation, with no significant change to energy resolution or hole mobility and lifetime. Additionally, many of the devices are still functional after being exposed to a 10 Mrad dose over 3 days, and those that do not survive can still be refabricated into working detectors. These results suggest that the failure mode in these devices is likely related to the interface between the electrode and material and their reaction, or the electrode itself and not the material itself. Overall, the study suggests that CsPbBr3 has high potential as a reliable and efficient radiation detector in various applications, including those involving extreme fluxes and energies of gamma-ray radiation.

Photoconduction Properties in Tungsten Disulfide Nanostructures

We reported the photoconduction properties of tungsten disulfide (WS2) nanoflakes obtained by the mechanical exfoliation method. The photocurrent measurements were carried out using a 532 nm laser source with different illumination powers. The results reveal a linear dependence of photocurrent on the excitation power, and the photoresponsivity shows an independent behavior at higher light intensities (400-4000 Wm-2). The WS2 photodetector exhibits superior performance with responsivity in the range of 36-73 AW-1 and a normalized gain in the range of 3.5-7.3 10-6 cm2V-1 at a lower bias voltage of 1 V. The admirable photoresponse at different light intensities suggests that WS2 nanostructures are of potential as a building block for novel optoelectronic device applications.

An Indium Synthetic Etioporphyrin for Organic Electronics: Aggregation and Photoconductivity in Thin Films

A new complex of indium(III)chloride with etioporphyrin-I was synthesized and characterized. As with naturally occurring extraligated etioporphyrins, the InCl-EtioP-I spectrum in solution has a very strong B-band and a more than an order of magnitude weaker Q-band, but this difference diminishes in solid films of InCl-EtioP-I obtained by thermal evaporation in vacuum. In a solid, molecules have a tight convex-convex arrangement in a 2D double layered structure with interplane distance of 3.066 Å. The conductivity of films can easily be activated by the action of temperature or light. In the cells with symmetrical lateral contacts the photocurrent exceeds the dark current by about three orders of magnitude, with the contribution of photons in the Q-band range being greater than expected from the experimental or calculated absorption spectrum. The Q-bands contribute significantly to the photovoltaic effect in the ITO/InCl-EtioP-I/Al sandwich cells. Such cells show an untypically strong signal in the photodiode regime, which yields the spectral detectivity of 10^12 Jones.

UV Sensor Based on Surface Acoustic Waves in ZnO/Fused Silica

Zinc oxide (ZnO) thin films have been grown by radio frequency sputtering technique on fused silica substrates. Optical and morphological characteristics of as-grown ZnO samples were measured by various techniques; an X-ray diffraction spectrum showed that the films exhibited hexagonal wurtzite structure and were c-axis-oriented normal to the substrate surface. Scanning electron microscopy images showed the dense columnar structure of the ZnO layers, and light absorption measurements allowed us to estimate the penetration depth of the optical radiation in the 200 to 480 nm wavelength range and the ZnO band-gap. ZnO layers were used as a basic material for surface acoustic wave (SAW) delay lines consisting of two Al interdigitated transducers (IDTs) photolithographically implemented on the surface of the piezoelectric layer. The Rayleigh wave propagation characteristics were tested in darkness and under incident UV light illumination from the top surface of the ZnO layer and from the fused silica/ZnO interface. The sensor response, i.e., the wave velocity shift due to the acoustoelectric interaction between the photogenerated charge carriers and the electric potential associated with the acoustic wave, was measured for different UV power densities. The reversibility and repeatability of the sensor responses were assessed. The time response of the UV sensor showed a rise time and a recovery time of about 10 and 13 s, respectively, and a sensitivity of about 318 and 341 ppm/(mW/cm²) for top and bottom illumination, respectively. The ZnO/fused silica-based SAW UV sensors can be interrogated across the fused silica substrate thanks to its optical transparency in the UV range. The backlighting interrogation can find applications in harsh environments, as it prevents the sensing photoconductive layer from aggressive environmental effects or from any damage caused by cleaning the surface from dust which could deteriorate the sensor's performance. Moreover, since the SAW sensors, by their operating principle, are suitable for wireless reading via radio signals, the ZnO/fused-silica-based sensors have the potential to be the first choice for UV sensing in harsh environments.

Fast Reconfigurable Electrode Array Based on Titanium Oxide for Localized Stimulation of Cultured Neural Network

Planar microelectrode arrays have become standard tools for in vitro neural-network analysis. However, these predefined micropatterned devices lack adaptability to target-specific cells within a cultured network. Herein, we fabricated a reconfigurable TiO₂ electrode array with an anatase-brookite bicrystalline polymorphous mesoporous layer. Because of its selective absorption of ultraviolet (UV) light and corresponding photoconductivity, TiO₂ electrode array was identified as a promising tool for high-resolution light-addressing. The TiO₂ film was used as a semitransparent semiconductor with a high R_off/R_on ratio of 10⁵ and a fast response time of 400 ms. In addition, the effect of UV radiation on the resistance of the TiO₂ film over 30 d in an aqueous environment was analyzed, with the film exhibiting high stability. An arbitrary UV pattern was applied to a reconfigurable TiO₂ electrode using a digital micromirror device (DMD), affording highly localized neural stimulation at the single-cell level. The reconfigurable TiO₂ electrode with a patterned indium tin oxide (ITO) substrate enabled the independent connection of up to 60 points with external stimulators and signal recorders. We believe this technique would be helpful for electrophysiological research requiring the analysis of cell and neural-network features using a highly localized neural interface.

Hydrothermal Synthesis of Ni3TeO6 and Cu3TeO6 Nanostructures for Magnetic and Photoconductivity Applications

Despite great attention toward transition metal tellurates especially M₃TeO₆ (M = transition metal) in magnetoelectric applications, control on single phasic morphology-oriented growth of these tellurates at the nanoscale is still missing. Herein, a hydrothermal synthesis is performed to synthesize single-phased nanocrystals of two metal tellurates, i.e., Ni₃TeO₆ (NTO with average particle size ∼37 nm) and Cu₃TeO₆ (CTO ∼ 140 nm), using NaOH as an additive. This method favors the synthesis of pure NTO and CTO nanoparticles without the incorporation of Na at pH = 7 in MTO crystal structures such as Na₂M₂TeO₆, as it happens in conventional synthesis approaches such as solid-state reaction and/or coprecipitation. Systematic characterization techniques utilizing in-house and synchrotron-based characterization methods for the morphological, structural, electronic, magnetic, and photoconductivity properties of nanomaterials showed the absence of Na in individual particulate single-phase MTO nanocrystals. Prepared MTO nanocrystals also exhibit slightly higher antiferromagnetic interactions (e.g., T _N-NTO = 57 K and T _N-CTO = 68 K) compared to previously reported MTO single crystals. Interestingly, NTO and CTO show not only a semiconducting nature but also photoconductivity. The proposed design scheme opens the door to any metal tellurates for controllable synthesis toward different applications. Moreover, the photoconductivity results of MTO nanomaterials prepared serve as a preliminary proof of concept for potential application as photodetectors.

The role of defects in the persistent photoconductivity of BaSnO3thin films

Time-dependent photoconductivity (PC) and PC spectra have been studied in oxygen deficient BaSnO₃thin films grown on different substrates. X-ray spectroscopy measurements show that the films have epitaxially grown on MgO and SrTiO₃substrates. While on MgO the films are nearly unstrained, on SrTiO₃the resulting film is compressive strained in the plane. Electrical conductivity in dark is increased in one order of magnitude for the films on SrTiO₃in comparison to the one on MgO. This leads to an increase of PC in the latter film in at least one order of magnitude. PC spectra show a direct gap with a value ofEG=3.9eV for the film grown on MgO while on SrTiO₃EG=3.36eV. For both type of films, time-dependent PC curves show a persistent behavior after illumination is removed. These curves have been fitted employing an analytical procedure based on the frame of PC as a transmission phenomenon showing the relevant role of donor and acceptor defects as carrier traps and as a source of carriers. This model also suggests that in the BaSnO₃film on SrTiO₃more defects are created probably due to strain. This latter effect can also explain the different transition values obtained for both type of films.

Thermal Effect during Laser-Induced Plasmonic Heating of Polyelectrolyte-Coated Gold Nanorods in Well Plates

We examined the generation and transfer of heat when laser irradiation is applied to water containing a suspension of gold nanorods coated with different polyelectrolytes. The ubiquitous well plate was used as the geometry for these studies. The predictions of a finite element model were compared to experimental measurements. It is found that relatively high fluences must be applied in order to generate biologically relevant changes in temperature. This is due to the significant lateral heat transfer from the sides of the well, which strongly limits the temperature that can be achieved. A 650 mW continuous-wave (CW) laser, with a wavelength that is similar to the longitudinal plasmon resonance peak of the gold nanorods, can deliver heat with an overall efficiency of up to 3%. This is double the efficiency achievable without the nanorods. An increase in temperature of up to 15 °C can be achieved, which is suitable for the induction of cell death by hyperthermia. The nature of the polymer coating on the surface of the gold nanorods is found to have a small effect.

Optically Controlling Broadband Terahertz Modulator Based on Layer-Dependent PtSe2 Nanofilms

In this paper, we propose an optically controlling broadband terahertz modulator of a layer-dependent PtSe₂ nanofilm based on a high-resistance silicon substrate. Through optical pump and terahertz probe system, the results show that compared with 6-, 10-, and 20-layer films, a 3-layer PtSe₂ nanofilm has better surface photoconductivity in the terahertz band and has a higher plasma frequency ω_p of 0.23 THz and a lower scattering time τ_s of 70 fs by Drude-Smith fitting. By the terahertz time-domain spectroscopy system, the broadband amplitude modulation of a 3-layer PtSe₂ film in the range of 0.1-1.6 THz was obtained, and the modulation depth reached 50.9% at a pump density of 2.5 W/cm². This work proves that PtSe₂ nanofilm devices are suitable for terahertz modulators.

Spatially Resolved Photo-Response of a Carbon Nanotube/Si Photodetector

Photodetectors based on vertical multi-walled carbon nanotube (MWCNT) film-Si heterojunctions are realized by growing MWCNTs on n-type Si substrates with a top surface covered by Si₃N₄ layers. Spatially resolved photocurrent measurements reveal that higher photo detection is achieved in regions with thinner MWCNT film, where nearly 100% external quantum efficiency is achieved. Hence, we propose a simple method based on the use of scotch tape with which to tune the thickness and density of as-grown MWCNT film and enhance device photo-response.

Terahertz Photoconductivity in Bilayer Graphene Transistors: Evidence for Tunneling at Gate-Induced Junctions

Photoconductivity of novel materials is the key property of interest for design of photodetectors, optical modulators, and switches. Despite the photoconductivity of most novel 2d materials having been studied both theoretically and experimentally, the same is not true for 2d p-n junctions that are necessary blocks of most electronic devices. Here, we study the sub-terahertz photocoductivity of gapped bilayer graphene with electrically induced p-n junctions. We find a strong positive contribution from junctions to resistance, temperature resistance coefficient, and photoresistivity at cryogenic temperatures T ∼ 20 K. The contribution to these quantities from junctions exceeds strongly the bulk values at uniform channel doping even at small band gaps of ∼10 meV. We further show that positive junction photoresistance is a hallmark of interband tunneling, and not of intraband thermionic conduction. Our results point to the possibility of creating various interband tunneling devices based on bilayer graphene, including steep-switching transistors and selective sensors.

Sensitization of ZnO Photoconductivity in the Visible Range by Colloidal Cesium Lead Halide Nanocrystals

In this work, colloidal perovskite nanocrystals (PNCs) are used to sensitize the photoconductivity of nanocrystalline ZnO films in the visible range. Nanocrystalline ZnO with a crystallite size of 12-16 nm was synthesized by precipitation of a zinc basic carbonate from an aqueous solution, followed by annealing at 300 °C. Perovskite oleic acid- and oleylamine-capped CsPbBr₃, CsPb(Cl/Br)₃ and CsPb(Br/I)₃ PNCs with a size of 6-13 nm were synthesized by a hot injection method at 170 °C in 1-octadecene. Photoconductive nanocomposites were prepared by applying a hexane sol of PNCs to a thick (100 μm) polycrystalline conductive ZnO layer. The spectral dependence of the photoconductivity, the dependence of the photoconductivity on irradiation, and the relaxation of the photoconductivity of the obtained nanocomposites have been studied. Sensitization of ZnO by CsPbBr₃ and CsPb(Cl/Br)₃ PNCs leads to enhanced photoconductivity in the visible range, the maximum of which is observed at 460 and 500 nm, respectively; close to the absorption maximum of PNCs. Nanocomposites ZnO/CsPb(Br/I)₃ turned out to be practically not photosensitive when irradiated with light in the visible range. The data obtained are discussed in terms of the position of the energy levels of ZnO and PNCs and the probable PNCs photodegradation. The structure, morphology, composition, and optical properties of the synthesized nanocrystals have also been studied by XRD, TEM, and XPS. The results can be applied to the creation of artificial neuromorphic systems in the visible optical range.

Colloidal Synthesis, Characterization, and Photoconductivity of Quasi-Layered CuCrS2 Nanosheets

The current need to accelerate the adoption of photovoltaic (PV) systems has increased the need to explore new nanomaterials that can harvest and convert solar energy into electricity. Transition metal dichalcogenides (TMDCs) are good candidates because of their tunable physical and chemical properties. CuCrS₂ has shown good electrical and thermoelectrical properties; however, its optical and photoconductivity properties remain unexplored. In this study, we synthesized CuCrS₂ nanosheets with average dimensions of 43.6 ± 6.7 nm in length and 25.6 ± 4.1 nm in width using a heat-up synthesis approach and fabricated films by the spray-coating method to probe their photoresponse. This method yielded CuCrS₂ nanosheets with an optical bandgap of ~1.21 eV. The fabricated film had an average thickness of ~570 nm, exhibiting a net current conversion efficiency of ~11.3%. These results demonstrate the potential use of CuCrS₂ as an absorber layer in solar cells.

Origin of Negative Photoconductivity at the Interface of Ba0.8Sr0.2TiO3/LaMnO3/Ba0.8Sr0.2TiO3 Heterostructures

The study of Ba_0.8Sr_0.2TiO₃/LaMnO₃/Ba_0.8Sr_0.2TiO₃ heterostructures on a MgO substrate with Ba_0.8Sr_0.2TiO₃ ferroelectric films revealed the occurrence of a metallic character of the temperature behavior of the resistance at a temperature less than 175 K. This behavior is associated with an increased charge concentration at the interface due to a discontinuity in the ferroelectric polarization at the interface between the films. At these temperatures, the effect of negative photoconductivity is observed under uniform illumination with the light of a selected spectral composition event on the surface of the ferroelectric film. The combined exposure to green and infrared light led to an addition of the effects. As a result, a cumulative effect was observed. The effect of metallic conductivity is due to the discontinuity of ferroelectric polarization. Therefore, we explain that the partial screening of the ferroelectric polarization by photogenerated charge carriers causes a reduction in the carrier concentration at the interface. Measurements in the Kelvin mode of atomic force microscopy showed that illumination influences the surface charge concentration in a similar way; this observation confirms our hypothesis.

Millimeter-Wave Permittivity Variations of an HR Silicon Substrate from the Photoconductive Effect

The photoinduced microwave complex permittivity of a highly resistive single-crystal silicon wafer was extracted from a bistatic free-space characterization test bench operating in the 26.5-40 GHz frequency band under CW optical illumination at wavelengths of 806 and 971 nm. Significant variations in the real and imaginary parts of the substrate's permittivity induced by direct photoconductivity are reported, with an optical power density dependence, in agreement with the theoretical predictions. These experimental results open the route to ultrafast system reconfiguration of microwave devices in integrated technology by an external EMI-protected and contactless control with unprecedented performance.

Near-IR Light-Tunable Omnidirectional Broadband Terahertz Wave Antireflection Based on a PEDOT:PSS/Graphene Hybrid Coating

Omnidirectional broadband terahertz (THz) antireflection (AR) with an actively configurable coating promises the achievement of next-generation efficient and versatile THz components with high performance. We demonstrate a near-infrared (NIR) light-tunable and omnidirectional broadband THz AR coating based on an impedance matching method and composed of a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/graphene composite film. The omnidirectional broadband properties of the active AR coating can be efficiently achieved by tunable NIR optical excitation of less than 0.27 W·cm^-2, which exhibits omnidirectional suppression of THz-wave reflection for incidence angles from 0 to 70°, concerning the broadband frequency range of 0.1-3.0 THz, with an ultrafast response time of ∼5 ps. Furthermore, we demonstrate that the active AR coating can improve the performance of a reflectance-tunable THz-wave polarization reflector by the elimination of Fabry-Pérot interference. The NIR irradiance-dependent active AR mechanism of the hybrid system is investigated, which demonstrates the essential role of the PEDOT:PSS/graphene layers in promoting the charge separation at the interface and therefore changing the photoconductivity of the composite film to achieve impedance matching under optical excitation. Several crucial advantages of the proposed and proven concept, including the wide-angle range, broad spectral range, flexible tunability, and easier fabrication, may revolutionize the AR strategy at THz frequencies for a wide range of THz applications.

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3-0.5%As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se_1-xAs_x alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

Enhanced Photoconductivity at Dislocations in SrTiO3

Dislocations are 1D crystallographic line defects and are usually seen as detrimental to the functional properties of classic semiconductors. It is shown here that this not necessarily accounts for oxide semiconductors in which dislocations are capable of boosting the photoconductivity. Strontium titanate single crystals are controllably deformed to generate a high density of ordered dislocations of two slip systems possessing different mesoscopic arrangements. For both slip systems, nanoscale conductive atomic force microscope investigations reveal a strong enhancement of the photoconductivity around the dislocation cores. Macroscopic in-plane measurements indicate that the two dislocation systems result in different global photoconductivity behavior despite the similar local enhancement. Depending on the arrangement, the global photoresponse can be increased by orders of magnitude. Additionally, indications for a bulk photovoltaic effect enabled by dislocation-surrounding strain fields are observed for the first time. This proves that dislocations in oxide semiconductors can be of large interest for tailoring photoelectric functionalities. Direct evidence that electronic transport is confined to the dislocation core points to a new emerging research field.

Probing Carrier Dynamics in sp3-Functionalized Single-Walled Carbon Nanotubes with Time-Resolved Terahertz Spectroscopy

The controlled introduction of covalent sp³ defects into semiconducting single-walled carbon nanotubes (SWCNTs) gives rise to exciton localization and red-shifted near-infrared luminescence. The single-photon emission characteristics of these functionalized SWCNTs make them interesting candidates for electrically driven quantum light sources. However, the impact of sp³ defects on the carrier dynamics and charge transport in carbon nanotubes remains an open question. Here, we use ultrafast, time-resolved optical-pump terahertz-probe spectroscopy as a direct and quantitative technique to investigate the microscopic and temperature-dependent charge transport properties of pristine and functionalized (6,5) SWCNTs in dispersions and thin films. We find that sp³ functionalization increases charge carrier scattering, thus reducing the intra-nanotube carrier mobility. In combination with electrical measurements of SWCNT network field-effect transistors, these data enable us to distinguish between contributions of intra-nanotube band transport, sp³ defect scattering and inter-nanotube carrier hopping to the overall charge transport properties of nanotube networks.

Synthesis and solar blind photosensitivity of crystalline boron nanowires

Herein, single crystalline boron nanowires (BNWs) have been synthesized by chemical vapor transport using boron element as boron source, iodine as transport agent, and Au as catalyst. The results demonstrate that BNWs can be all formed at 600 °C-950 °C for 2 h, and possess rhombohedral crystal structure (β-boron). The NWs have diameters from several to hundreds of nanometers, and lengths from several to hundreds of microns. A single nanowire has been fabricated to field effect transistor (FET) which shows excellent solar blind photosensitivity and selectivity. The photo/dark current ratio and photoresponsitity is 1.14 and 97.6 mA W^-1at a bias of 5 V under light illumination of 254 nm with 0.42 mW cm^-2, respectively, and both the rising and decay time of the on-off currents are 4.6 s and 10.3 s, respectively. When the FET is used as a personal breath sensor, the ratio of exsufflating and inhaling currents is 2.7, rising and decay time of the breath currents are 0.4 s and 2.2 s, respectively. So the BNWs are important sense materials.

Breaking the Cut-Off Wavelength Limit of GaTe through Self-Driven Oxygen Intercalation in Air

Low symmetric two dimensional (2D) semiconductors are of great significance for their potential applications in polarization-sensitive photodetection and quantum information devices. However, their real applications are limited by their photo-detecting wavelength ranges, which are restricted by their fundamental optical bandgaps. Recently, intercalation has been demonstrated to be a powerful strategy to modulate the optical bandgaps of 2D semiconductors. Here, the authors report the self-driven oxygen (O₂ ) intercalation induced bandgap reduction from 1.75 to 1.19 eV in gallium telluride (GaTe) in air. This bandgap shrinkage provides the long-wavelength detection threshold above ≈1100 nm for O₂ intercalated GaTe (referred to as GaTeO₂ ), well beyond the cut-off wavelength at ≈708 nm for pristine GaTe. The GaTeO₂ photodetectors have a high photoresponsivity, and highly anisotropic photodetection behavior to even sub-waveband radiation. The dichroic ratio (I_max /I_min ) of photocurrent is about 1.39 and 2.9 for 600 nm and 1100 nm, respectively. This findings demonstrates a broadband photodetector utilizing GaTe after breaking through its bandgap limitation by self-driven O₂ intercalation in air and further reveal its photoconductivity anisotropic nature. This provides design strategies of 2D materials-based high-performance broadband photodetectors for the exploration of polarized state information.

Orbital Gating Driven by Giant Stark Effect in Tunneling Phototransistors

Conventional gating in transistors uses electric fields through external dielectrics that require complex fabrication processes. Various optoelectronic devices deploy photogating by electric fields from trapped charges in neighbor nanoparticles or dielectrics under light illumination. Orbital gating driven by giant Stark effect is demonstrated in tunneling phototransistors based on 2H-MoTe₂ without using external gating bias or slow charge trapping dynamics in photogating. The original self-gating by light illumination modulates the interlayer potential gradient by switching on and off the giant Stark effect where the d_z 2-orbitals of molybdenum atoms play the dominant role. The orbital gating shifts the electronic bands of the top atomic layer of the MoTe₂ by up to 100 meV, which is equivalent to modulation of a carrier density of 7.3 × 10¹¹ cm^-2 by electrical gating. Suppressing conventional photoconductivity, the orbital gating in tunneling phototransistors achieves low dark current, practical photoresponsivity (3357 AW^-1 ), and fast switching time (0.5 ms) simultaneously.

Reversible Manipulation of Photoconductivity Caused by Surface Oxygen Vacancies in Perovskite Stannates with Ultraviolet Light

Programmable optoelectronic devices call for the reversible control of the photocarrier recombination process by in-gap states in oxide semiconductors. However, previous approaches to produce oxygen vacancies as a source of in-gap states in oxide semiconductors have hampered the reversible formation of oxygen vacancies and their related phenomena. Here, a new strategy to manipulate the 2D photoconductivity from perovskite stannates is demonstrated by exploiting spatially selective photochemical reaction under ultraviolet illumination at room temperature. Remarkably, the ideal trap-free photocurrent of air-illuminated BaSnO₃ (≈200 pA) is reversibly switched into three orders of magnitude higher photocurrent of vacuum-illuminated BaSnO₃ (≈335 nA) with persistent photoconductivity depending on ambient oxygen pressure under illumination. Multiple characterizations elucidate that ultraviolet illumination of BaSnO₃  under low oxygen pressure induces surface oxygen vacancies as a result of surface photolysis combined with the low oxygen-diffusion coefficient of BaSnO₃ ; the concentrated oxygen vacancies are likely to induce a two-step transition of photocurrent response by changing the characteristics of in-gap states from the shallow level to the deep level. These results suggest a novel strategy that uses light-matter interaction in a reversible and spatially confined way to manipulate functionalities related to surface defect states, for the emerging applications using newly discovered oxide semiconductors.

Photoconductivity of Thin Films Obtained from a New Type of Polyindole

The optoelectronic properties of a new poly(2-ethyl-3-methylindole) (MPIn) are discussed in this paper. The absorption and photoluminescence spectra were studied. The electronic spectrum of MPIn showed a single absorption maximum at 269 nm that is characteristic of the entire series of polyindoles. The fluorescence spectra show that the emission peaks of the test sample are centered around 520 nm. The photoconductivity of thin film samples of MPIn polyindole was studied by measuring the current-voltage characteristics under ultraviolet radiation with a wavelength of 350 nm. Samples of phototransistors were obtained, where thin films of MPIn polyindole were used as a transport layer, and their characteristics were measured and analyzed. The value of the quantum efficiency and the values of the mobility of charge carriers in thin polyindole films were estimated.

Photocatalytic Mechanism Control and Study of Carrier Dynamics in CdS@C3N5 Core-Shell Nanowires

We present a potential solution to the problem of extraction of photogenerated holes from CdS nanocrystals and nanowires. The nanosheet form of C₃N₅ is a low-band-gap (E_g = 2.03 eV), azo-linked graphenic carbon nitride framework formed by the polymerization of melem hydrazine (MHP). C₃N₅ nanosheets were either wrapped around CdS nanorods (NRs) following the synthesis of pristine chalcogenide or intercalated among them by an in situ synthesis protocol to form two kinds of heterostructures, CdS-MHP and CdS-MHPINS, respectively. CdS-MHP improved the photocatalytic degradation rate of 4-nitrophenol by nearly an order of magnitude in comparison to bare CdS NRs. CdS-MHP also enhanced the sunlight-driven photocatalytic activity of bare CdS NWs for the decolorization of rhodamine B (RhB) by a remarkable 300% through the improved extraction and utilization of photogenerated holes due to surface passivation. More interestingly, CdS-MHP provided reaction pathway control over RhB degradation. In the absence of scavengers, CdS-MHP degraded RhB through the N-deethylation pathway. When either hole scavenger or electron scavenger was added to the RhB solution, the photocatalytic activity of CdS-MHP remained mostly unchanged, while the degradation mechanism shifted to the chromophore cleavage (cycloreversion) pathway. We investigated the optoelectronic properties of CdS-C₃N₅ heterojunctions using density functional theory (DFT) simulations, finite difference time domain (FDTD) simulations, time-resolved terahertz spectroscopy (TRTS), and photoconductivity measurements. TRTS indicated high carrier mobilities >450 cm² V^-1 s^-1 and carrier relaxation times >60 ps for CdS-MHP, while CdS-MHPINS exhibited much lower mobilities <150 cm² V^-1 s^-1 and short carrier relaxation times <20 ps. Hysteresis in the photoconductive J-V characteristics of CdS NWs disappeared in CdS-MHP, confirming surface passivation. Dispersion-corrected DFT simulations indicated a delocalized HOMO and a LUMO localized on C₃N₅ in CdS-MHP. C₃N₅, with its extended π-conjugation and low band gap, can function as a shuttle to extract carriers and excitons in nanostructured heterojunctions, and enhance performance in optoelectronic devices. Our results demonstrate how carrier dynamics in core-shell heterostructures can be manipulated to achieve control over the reaction mechanism in photocatalysis.

Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double-Helix SnIP Nanowires

Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, the understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, time-resolved terahertz (THz) spectroscopy is used to probe the transient photoconductivity of SnIP nanowire films and measure the carrier mobility. With insight into the highly anisotropic electronic structure from quantum chemical calculations, an electron mobility as high as 280 cm²   V^-1 s^-1 along the double-helix axis and a hole mobility of 238 cm²   V^-1 s^-1 perpendicular to the double-helix axis are detected. Additionally, infrared-active (IR-active) THz vibrational modes are measured, which shows excellent agreement with first-principles calculations, and an ultrafast photoexcitation-induced charge redistribution is observed that reduces the amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, it is shown that the carrier lifetime and mobility are limited by a trap density greater than 10¹⁸  cm^-3 . The results provide insight into the optical excitation and relaxation pathways of SnIP and demonstrate a remarkably high carrier mobility for such a soft and flexible material, suggesting that it could be ideally suited for flexible electronics applications.

Ultrafast photocarrier dynamics in Fe-implanted InGaAs polycrystalline photoconductive materials

We investigate the ultrafast photoconductivity and charge-carrier transport in thermally annealed Fe-implanted InGaAs/InP films using time-resolved terahertz spectroscopy. The samples were fabricated from crystalline InGaAs films amorphized with Fe ions implantation. The rapid thermal annealing of the InGaAs layer induces solid recrystallization through the formation of polycrystalline grains whose sizes are shown to increase with increasing annealing temperature within the 300-700 °C range. Based on the influence of the laser fluence, the temporal profile of the time-resolved photoconductivity was reproduced using a system of rate equations that describe the photocarrier dynamics in terms of a capture/recombination mechanism. For annealing temperatures below 500 °C, the capture time is found to be less than 1 ps while the recombination time from the charged states did not exceed 5 ps. However, for higher annealing temperatures, the capture and the recombination times show a continuous increase, reaching 7.1 ps and 1 ns respectively, for the film annealed at 700 °C. Frequency-dependent photoconductivity curves are analyzed via a modified Drude-Smith model that considers a diffusive restoring current and the confining particles' sizes. Our results demonstrate that the localization parameter of the photocarrier transport model is correlated to the polycrystalline grain size. We also show that a relatively high effective mobility of about 2570 cm² V^-1 s^-1is preserved in all these Fe-implanted InGaAs films.

Experimental and Theoretical Investigation of the Structural and Opto-electronic Properties of Fe-Doped Lead-Free Cs2 AgBiCl6 Double Perovskite

Lead-free double perovskites have emerged as stable and non-toxic alternatives to Pb-halide perovskites. Herein, the synthesis of Fe-doped Cs2 AgBiCl6 lead-free double perovskites are reported that display blue emission using an antisolvent method. The crystal structure, morphology, optical properties, band structure, and stability of the Fe-doped double perovskites were investigated systematically. Formation of the Fe-doped Cs2 AgBiCl6 double perovskite is confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis. XRD and thermo-gravimetric analysis (TGA) shows that the Cs2 AgBiCl6 double perovskite has high structural and thermal stability, respectively. Field emission scanning electron microscopy (FE-SEM) analysis revealed the formation of dipyramidal shape Cs2 AgBiCl6 crystals. Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping shows the overlapping of Cs, Bi, Ag, Fe, and Cl elements and homogenous incorporation of Fe in Cs2 AgBiCl6 double perovskite. The Fe-doped Cs2 AgBiCl6 double perovskite shows a strong absorption at 380 nm. It extends up to 700 nm, suggesting that sub-band gap states transition may originate from the surface defect of the doped perovskite material. The radiative kinetics of the crystals was studied using the time-correlated single-photon counting (TCSPC) technique. Lattice parameters and band gap value of the Fe-doped Cs2 AgBiCl6 double perovskites predicted by the density functional theory (DFT) calculations are confirmed by XRD and UV/Visible spectroscopy analysis. Time-dependent photo-response characteristics of the Fe-doped Cs2 AgBiCl6 double perovskite show fast response and recovery time of charge carriers. We believe that the successful incorporation of Fe in lead-free, environmentally friendly Cs2 AgBiCl6 double perovskite can open a new class of doped double perovskites with significant potential optoelectronics devices fabrication and photocatalytic applications.

Exciton Transport in Molecular Semiconductor Crystals for Spin-Optoelectronics Paradigm

Organic semiconductors with long-range exciton diffusion length are highly desirable for optoelectronics but currently remain rare. Here, the estimated diffusion length of singlet excitons (L_D ) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent evaporation was shown to be up to approximately 124 nm. These crystals showed a previously unseen parallelogram morphology with layer-by-layer edge-on molecular stacking, isotropic optical waveguiding, radiation rate and non-radiation rate constants of 0.15 and 0.26 ns^-1 respectively, as well as good field-effect transistor hole mobility and theoretically computed strong electronic couplings as high as 109 meV. Photoresponse experiments revealed that the photoconductivity of DPA crystals is surprisingly not related to the radiative pathway but associated with rapid exciton diffusion to the crystal surface for charge separation and carrier bimolecular recombination. Taken together, DPA was shown to be a promising semiconducting material for a new organic optoelectronics paradigm.

Modification of the photoconducting properties of ZnO thin films via low-temperature annealing and air exposure

A simple thermal annealing at 150 °C followed by exposure to air ambient conditions in epitaxial ZnO thin films produces a photoconductivity enhancement and a reduction of the energy gap. The first effect is related to a release of carriers from bulk traps while the second is caused by a gradual adsorption of species on the film surface which increases the band bending, as x-ray photoemission spectroscopy (XPS) shows. An observed drift of the photoconductivity and the energy gap over the days is connected to this adsorption kinetics. These findings have a potential application in ZnO based optoelectronic devices.

Zn-Al Layered Double Hydroxide Film Functionalized by a Luminescent Octahedral Molybdenum Cluster: Ultraviolet-Visible Photoconductivity Response

A novel UV-Vis photodetector consisting of an octahedral molybdenum cluster-functionalized Zn₂Al layered double hydroxide (LDH) has been successfully synthesized by co-precipitation and delamination methods under ambient conditions. The electrophoretic deposition process has been used as a low-cost, fast, and effective method to fabricate thin and transparent nanocomposite films containing a dense and regular layered structure. The study provided evidence that the presence of the Mo₆ cluster units between the LDH does not affect the ionic conduction mechanism of the LDH, which linearly depends on the relative humidity and temperature. Moreover, the photocurrent response is remarkably extended to the visible domain. The reproducibility and stabilization of the photocurrent response caused by the Mo₆ cluster-functionalized LDH have been verified upon light excitation at 540 nm. Additionally, it was demonstrated that the films show advantageously strong adherence properties for application requirements.

Conductivity and Photoconductivity of a p-Type Organic Semiconductor under Ultrastrong Coupling

During the past decade, it has been shown that light-matter strong coupling of materials can lead to modified and often improved properties which has stimulated considerable interest. While charge transport can be enhanced in n-type organic semiconductors by coupling the electronic transition and thereby splitting the conduction band into polaritonic states, it is not clear whether the same process can also influence carrier transport in the valence band of p-type semiconductors. Here we demonstrate that it is indeed possible to enhance both the conductivity and photoconductivity of a p-type semiconductor rr-P3HT that is ultrastrongly coupled to plasmonic modes. It is due to the hybrid light-matter character of the virtual polaritonic excitations affecting the linear response of the material. Furthermore, in addition to being enhanced, the photoconductivity of rr-P3HT shows a modified spectral response due to the formation of the hybrid polaritonic states. This illustrates the potential of engineering the vacuum electromagnetic environment to improve the optoelectronic properties of organic materials.

Phase Controlled Growth of Cd3As2 Nanowires and Their Negative Photoconductivity

The bottom-up synthesis process often allows the growth of metastable phase nanowires instead of the thermodynamically stable phase. Herein, we synthesized Cd₃As₂ nanowires with a controlled three-dimensional Dirac semimetal phase using a chemical vapor transport method. Three different phases such as the body centered tetragonal (bct), and two metastable primitive tetragonal (P4₂/nbc and P4₂/nmc) phases were identified. The conversion between three phases (bct → P4₂/nbc → P4₂/nmc) was achieved by increasing the growth temperature. The growth direction is [110] for bct and P4₂/nbc and [100] for P4₂/nmc, corresponding to the same crystallographic axis. Field effect transistors and photodetector devices showed the nearly same electrical and photoelectrical properties for three phases. Differential conductance measurement confirms excellent electron mobility (2 × 10⁴ cm²/(V s) at 10 K). Negative photoconductance was first observed, and the photoresponsivity reached 3 × 10⁴ A/W, which is ascribed to the surface defects acting as trap sites for the photogenerated electrons.

Photochromic Conjugated Microporous Polymer Manifesting Bio-Inspired pcFRET and Logic Gate Functioning

Design and synthesis of solid-state photochromic materials remain a challenge because of high structural constrain. However, this can be mitigated in attaining structural flexibility by introducing permanent porosity into the system. Here, we report for the first time the design and synthesis of a photochromic conjugated microporous polymer (pcCMP) by assembling photochromic dithienylethene aldehyde and benzene-1,3,5-tricarbohydrazide. The yellow photo-isomer pcCMP-O gets converted to a deep-green photo-isomer pcCMP-C by UV-light irradiation, which can be reverted to pcCMP-O by visible light or thermal treatment. Owing to the thermo-irreversible nature, the pcCMP is found to be suitable for designing an INH functioning logic gate. pcCMP-C shows highly enhanced conductivity (92 times) because of enhanced conjugation compared to pcCMP-O. Furthermore, we demonstrate the bio-inspired photo-switchable pcFRET process by encapsulation of a red-emissive green fluorescent protein (gfp) chromophore analogue into the pcCMP. This material shows high processibility and has been exploited further for secret writing.

Highly Crystalline and Semiconducting Imine-Based Two-Dimensional Polymers Enabled by Interfacial Synthesis

Single-layer and multi-layer 2D polyimine films have been achieved through interfacial synthesis methods. However, it remains a great challenge to achieve the maximum degree of crystallinity in the 2D polyimines, which largely limits the long-range transport properties. Here we employ a surfactant-monolayer-assisted interfacial synthesis (SMAIS) method for the successful preparation of porphyrin and triazine containing polyimine-based 2D polymer (PI-2DP) films with square and hexagonal lattices, respectively. The synthetic PI-2DP films are featured with polycrystalline multilayers with tunable thickness from 6 to 200 nm and large crystalline domains (100-150 nm in size). Intrigued by high crystallinity and the presence of electroactive porphyrin moieties, the optoelectronic properties of PI-2DP are investigated by time-resolved terahertz spectroscopy. Typically, the porphyrin-based PI-2DP 1 film exhibits a p-type semiconductor behavior with a band gap of 1.38 eV and hole mobility as high as 0.01 cm2  V-1  s-1 , superior to the previously reported polyimine based materials.

Bias Tunable Photocurrent in Metal-Insulator-Semiconductor Heterostructures with Photoresponse Enhanced by Carbon Nanotubes

Metal-insulator-semiconductor-insulator-metal (MISIM) heterostructures, with rectifying current-voltage characteristics and photosensitivity in the visible and near-infrared spectra, are fabricated and studied. It is shown that the photocurrent can be enhanced by adding a multi-walled carbon nanotube film in the contact region to achieve a responsivity higher than 100 &nbsp; mA &nbsp; W - 1 under incandescent light of 0.1 &nbsp; mW &nbsp; cm - 2 . The optoelectrical characteristics of the MISIM heterostructures are investigated at lower and higher biases and are explained by a band model based on two asymmetric back-to-back Schottky barriers. The forward current of the heterojunctions is due to majority-carrier injection over the lower barrier, while the reverse current exhibits two different conduction regimes corresponding to the diffusion of thermal/photo generated carriers and majority-carrier tunneling through the higher Schottky barrier. The two conduction regimes in reverse bias generate two plateaus, over which the photocurrent increases linearly with the light intensity that endows the detector with bias-controlled photocurrent.

Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultraflat Metals

Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS₂ immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.

Photoconductive Core-Shell Liquid-Crystals of a Perylene Bisimide J-Aggregate Donor-Acceptor Dyad

A novel core-shell structured columnar liquid crystal composed of a donor-acceptor dyad of tetraphenoxy perylene bisimide (PBI), decorated with four bithiophene units on the periphery, was synthesized. This molecule self-assembles in solution into helical J-aggregates guided by π-π interactions and hydrogen bonds which organize into a liquid-crystalline (LC) columnar hexagonal domain in the solid state. Donor and acceptor moieties exhibit contrasting exciton coupling behavior with the PBIs' (J-type) transition dipole moment parallel and the bithiophene side arms' (H-type) perpendicular to the columnar axis. The dyad shows efficient energy and electron transfer in solution as well as in the solid state. The synergy of photoinduced electron transfer (PET) and charge transport along the narcissistically self-assembled core-shell structure enables the implementation of the dye in two-contact photoconductivity devices giving rise to a 20-fold increased photoresponse compared to a reference dye without bithiophene donor moieties.

Photoresponsive Porphyrin Nanotubes of Meso-tetra(4-Sulfonatophenyl)Porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin

Porphyrin macrocycles and their supramolecular nanoassemblies are being widely explored in energy harvesting, sensor development, catalysis, and medicine because of a good tunability of their light-induced charge separation and electron/energy transfer properties. In the present work, we prepared and studied photoresponsive porphyrin nanotubes formed by the self-assembly of meso-tetrakis(4-sulfonatophenyl)porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin. Scanning electron microscopy and transmission electron microscopy showed that these tubular nanostructures were hollow with open ends and their length was 0.4–0.8 μm, the inner diameter was 7–15 nm, and the outer diameter was 30–70 nm. Porphyrin tectons, H₄TPPS42- : Sn(IV)TPyP⁴⁺, self-assemble into the nanotubes in a ratio of 2:1, respectively, as determined by the elemental analysis. The photoconductivity of the porphyrin nanotubes was determined to be as high as 3.1 × 10⁻⁴ S m⁻¹, and the dependence of the photoconductance on distance and temperature was investigated. Excitation of the Q-band region with a Q-band of SnTPyP⁴⁺ (550–552 nm) and the band at 714 nm, which is associated with J-aggregation, was responsible for about 34 % of the photoconductive activity of the H₄TPPS42--Sn(IV)TPyP⁴⁺ porphyrin nanotubes. The sensor properties of the H₄TPPS42-- Sn(IV)TPyP⁴⁺ nanotubes in the presence of iodine vapor and salicylate anions down to millimolar range were examined in a chemiresistor sensing mode. We have shown that the porphyrin nanotubes advantageously combine the characteristics of a sensor and a transducer, thus demonstrating their great potential as efficient functional layers for sensing devices and biomimetic nanoarchitectures.

An Integrated Germanium-Based THz Impulse Radiator with an Optical Waveguide Coupled Photoconductive Switch in Silicon

This paper presents an integrated germanium (Ge)-based THz impulse radiator with an optical waveguide coupled photoconductive switch in a low-cost silicon-on-insulator (SOI) process. This process provides a Ge thin film, which is used as photoconductive material. To generate short THz impulses, N++ implant is added to the Ge thin film to reduce its photo-carrier lifetime to sub-picosecond for faster transient response. A bow-tie antenna is designed and connected to the photoconductive switch for radiation. To improve radiation efficiency, a silicon lens is attached to the substrate-side of the chip. This design features an optical-waveguide-enabled “horizontal” coupling mechanism between the optical excitation signal and the photoconductive switch. The THz emitter prototype works with 1550 nm femtosecond lasers. The radiated THz impulses achieve a full-width at half maximum (FWHM) of 1.14 ps and a bandwidth of 1.5 THz. The average radiated power is 0.337 μW. Compared with conventional THz photoconductive antennas (PCAs), this design exhibits several advantages: First, it uses silicon-based technology, which reduces the fabrication cost; second, the excitation wavelength is 1550 nm, at which various low-cost laser sources operate; and third, in this design, the monolithic excitation mechanism between the excitation laser and the photoconductive switch enables on-chip programmable control of excitation signals for THz beam-steering.